The present invention relates to a laser beam scanning device in which a plurality of laser beams subjected to luminance modulation by image signals are deflected in a main scanning direction so as to be, respectively, irradiated onto members to be irradiated. The laser beam scanning device is provided in an image forming apparatus, for example, a laser printer so as to be used as an exposure means for photosensitive members, etc.
Conventionally, such image forming apparatuses as a printer, a display unit, etc. incorporate a scanning device which modulates luminance of light by image signals so as to irradiate the light onto a member to be irradiated, such as a photosensitive member, a photographic film, a screen or the like. A laser beam source, which can obtain a minute light spot and is capable of performing high-speed modulation, is employed as a light source of the known scanning device. Meanwhile, in order to deflect in a main scanning direction, a light spot irradiated onto the member to be irradiated, such deflection devices are known in which a polygon mirror is rotated, a galvanomirror is oscillated and a hologram is rotated. In the known deflection devices, the deflection device employing the polygon mirror is simpler, in structure, than the remaining deflection devices and therefore, is used most popularly.
Meanwhile, in order to simplify structure and reduce production cost of a known high-speed printer or color printer employing a plurality of laser beams, the polygon mirror is used in common with expensive lenses such that a plurality of laser beams are deflected simultaneously by the single polygon mirror.
Also in such known laser beam scanning devices, a beam detector is usually provided in the vicinity of the members to be irradiated in order to determine timing of start of modulation of laser beam sources effected by image signals at the time of deflection and the deflected laser beams scan the photosensitive members after passing through the beam detector.
An arrangement in which one beam detector is provided so as to determine timing of start of modulation of a plurality of laser beams is disclosed in such prior art documents as Japanese Patent Laid-Open Publication Nos. 67375/1982 and 104572/1981. In the first prior art document, a slit for preventing a plurality of the laser beams from being irradiated onto the beam detector at a time is provided in front of the beam detector such that the beam detector determines timing of start of modulation of each of the laser beams effected by image signals, on the basis of an output of the beam detector indicative of detection of passage of each of the laser beams. Meanwhile, in the second prior art document, on-off control of the laser beam sources is performed such that one laser beam is irradiated onto the beam detector. In the second prior art document, timing of modulation of each of the laser beams effected by the image signals is determined from interrelationship of the laser beams decided uniquely on the basis of an output of the beam detector indicative of detection of passage of the one laser beam.
However, in the first prior art document, a light receiving area of the beam detector is restricted by the slit. Thus, if scanning speed is raised due to responsiveness of the beam detector, such a problem arises that it becomes difficult to distinguish the laser beams from each other. Meanwhile, in the second prior art document, such inconveniences are incurred that not only a special on-off control circuit for turning on only one laser beam source and turning off the remaining laser beam sources during passage of spots of the laser beams in front of the beam detector is required to be provided but service life of the one laser beam source becomes shorter than the remaining laser beam sources.